Piranha

[Britney]

A woman that fights or grapples (especially a practitioner of Brazilian Jiu-Jitsu) that is known for being very fast and vicious.

[Dianne]

A [voracious], [predatory], highly [competitive], [ruthless], [alpha] male/female.
Often misspelt [Pirhana]

[Magaret]

adj: A determined, sneaky and fast opponent that is very hard to get rid of once they acquire their target.

[Julian]

noun: Any wrestling or grappling technique (esp from Brazilian Jiu-Jitsu) that presses a body part or the ground against your mouth - sometimes yielding imprints of your teeth on the inside of your lips/gums or teeth marks on your opponents arms/legs.

[Hina Muqadas]

1.s. f., Ictiol., Brasil,
teleósteo water fish cutting tooth candy, characterized for an extreme voracity and that it can constitute a real danger in the rivers and lakes where lives, therefore demonstrates great predilecção for animals that bleed;
2.Ornit.,
Brasilian black bird of Amazon;
3.gír.,
woman of libertine habits.

[Tanya Shivari]

In internet usage, particularly forum message boards, a user who swarms a post with useless commentary. Not considered a [spam], as the user will usually post as part of a larger group of users, and the said user will usually think their comment is complimentary or in some way helpful, even though it is not.
The comment is:

Usually worthless. OMG, thats great! is not as complimentary as they poster believes it to be, and only serves to waste the time of the original poster.

Not well thought out. The comment is usually quick, and does not justify itself. No reason is given as to why the user feels the way he / she does about said topic, image, story, etc.

Part of a mob mentality. Piranhas dont work alone, unlike most spammers. Their posts are usually followed by a swarm of like-minded posts, fueling the useless flood of empty comments.

NOTE: These types of comments are usually most prevelant on a forum centered around skills development. (Art sites, writing boards, game modification boards, etc.) Where a person skilled in whatever the forum centers on is easily attacked by piranhas and his / her post is quickly devoured.

[Gregory]

A member of Piranhas-Beach. A dragonboat club in Toronto

[Luis Fok]

A fish that eats everything.

[Oralee]

A deep-bodied South American freshwater fish that typically lives in shoals and has very sharp teeth that are used to tear flesh from prey. It has a reputation as a fearsome predator.